1. Field of the Invention
The present invention relates to a distortion compensating circuit, a power amplifier using the same, and a communication device having the power amplifier. In particular, the present invention relates to a distortion compensating circuit used in a wireless communication device or the like requiring low-distortion amplification, a power amplifier using the same, and the communication device.
2. Description of the Background Art
In current wireless communication systems such as a wireless LAN (Local Area Network) system and a mobile phone, a dominant technology is digital modulation and demodulation such as OFDM (Orthogonal Frequency Division Multiplex) and QPSK (Quadrature Phase Shift Keying). In these digital modulation and demodulation schemes, information is carried by using both a phase and an amplitude of a signal, and thus a power amplifier is required to linearly amplify an input signal. Further, in a power amplifier for transmission consuming much of power consumption of the system during transmission, the power amplifier is also required to operate with high efficiency in order to achieve system miniaturization and low power consumption.
As a technique for enhancing linearity of a power amplifier, there has been proposed a method of canceling distortion by providing, at a preceding stage or a subsequent stage of a power amplifying element used in a power amplifier, a distortion compensating circuit having nonlinearity contrary to the power amplifying element. As an example of a conventional distortion compensating circuit, a technique for connecting a circuit including a diode between a signal path and a ground has been disclosed (see for example Japanese Patent Laying-Open No. 2001-144550).
FIG. 9 is a circuit block diagram of a conventional distortion compensating circuit 100.
Referring to FIG. 9, conventional distortion compensating circuit 100 has an input terminal 101, capacitors 102, 103 and 110, an output terminal 104, a diode 105, resistors 106 and 108, and a direct-current power source 109.
Diode 105 has an anode terminal electrically coupled to a connecting node between capacitor 102 and capacitor 103, and a cathode terminal grounded via resistor 106. Direct-current power source 109 is connected to the anode. terminal of diode 105 via resistor 108, and a forward bias voltage is applied to diode 105. Capacitor 110 is connected to a connecting node between resistor 108 and direct-current power source 109, and the connecting node is grounded via capacitor 110.
Hereinafter, an operation of conventional distortion compensating circuit 100 will be described.
A signal input from input terminal 101 is output to output terminal 104 via capacitors 102 and 103. However, a portion of the signal leaks to ground via diode 105 and resistor 106 connected between capacitor 102 and capacitor 103, and via resistor 108 and capacitor 110. Since an increase in the input signal leads to increased power of the signal input to diode 105, a flow of direct current is produced by the rectification function of the diode, and a bias point of the diode moves.
FIG. 10 illustrates the movement of the bias point in conventional distortion compensating circuit 100.
Referring to FIG. 10, a curve 111 indicates a current-voltage characteristic of a diode when a signal is not input. When an input signal is small, the diode operates on a small-signal bias point S′ on curve 111. On the other hand, when input power is increased, a flow of direct current is produced by the rectification function of the diode as described above, and the current-voltage characteristic of the diode changes from curve 111 to a curve 112.
As a result, the bias point moves from small-signal bias point S′ to a large-signal bias point L′, along a load line 113 determined by resistors 106 and 108.
A resistance component of a diode with respect to high-frequency power is represented by an RF (radio frequency) resistance value, which corresponds to the reciprocal of the slope of a tangent to the curve indicating a current-voltage characteristic at a bias point. When power of a signal input to the diode is increased, the bias point moves from point S′ to point L′, and the tangent has a gentler slope, as shown in FIG. 10. That is, the RF resistance value of the diode increases. Accordingly, the amount of signal power leak to diode 105 is reduced with the increase in the input power, and as a result, output power from output terminal 104 increases with the increase in the input power. FIG. 11 shows this result.
FIG. 11 illustrates relationship between input power and gain, that is, power ratio between an output signal and an input signal, of a conventional distortion compensating circuit.
As shown in FIG. 11, the amount of gain reduction decreases with an increase in the input power. This indicates that the distortion compensating circuit has positive gain deviation, that is, it has a function of suppressing negative gain deviation. The positive gain deviation in the distortion compensating circuit can be adjusted by a power supply voltage of direct-current power source 109, which has been disclosed for example in Japanese Patent No. 3335907. The negative gain deviation can be suppressed by reducing the power supply voltage as shown in FIG. 11.
It is generally know that, to operate a power amplifier with high efficiency, a direct bias current in a power amplifying element used in the power amplifier should be reduced.
If the direct bias current is reduced, however, gain of the power amplifying element tends to increase with an increase in output power before the output power is saturated.
FIG. 12 illustrates relationship between output power and both gain and power efficiency of a power amplifying element.
As shown in FIG. 12, gain variation associated with an increase in the output power means that the linearity of a power amplifier is deteriorated. FIG. 12 indicates that improving the power efficiency and improving the linearity are in trade-off relationship.
Therefore, in order to operate a power amplifier with high linearity and with high efficiency, a distortion compensating circuit is required which can suppress gain increase caused when a direct bias current in a power amplifying element is reduced, that is, positive gain deviation.
However, since a distortion compensating circuit according to the above conventional technique has positive gain deviation, there has been a problem that it cannot be applied as a distortion compensating circuit for a power amplifying element having positive gain deviation.